1. Field of the Invention
The present invention relates to a method and apparatus for etching semiconductor materials.
2. Description of the Related Art
Many devices fabricated from silicon and other semiconductors must be truly three-dimensional. Some examples of these three-dimensional devices include micromechanical sensors and actuators, electrostatically driven micro-motors, and valve systems. Other three-dimensional structures are optical gratings and lenses, isolation trenches, mesas and via holes.
Three-dimensional structures formed from silicon and other semiconductor substrates are generally produced by anisotropic etching processes. At present, silicon is usually removed by simple chemical etching, that is, by immersing silicon specimens in a liquid bath of the chemical etchants. These are usually hydrofluoric solutions, strong alkalis such as KOH, or ethylene diamine-pirocathecol (EDP).
There are several limitations inherent in the etching techniques presently known and used in the art. The principal problem is a lack of control over the rate of silicon dissolution during the etching process. This lack of control restricts the utility of the micromachining methods presently known and used in the art.
Furthermore, at present, there is only one electrochemical technique which is reasonably effective in increasing the rate of silicon etching. This technique combines anodic biasing in aggressive HF solutions with illumination. However, the use of etchants like hydrofluoric acid, which are highly corrosive and environmentally hazardous, gives rise to serious problems in production and waste disposal.
In contrast to etching in HF, etching of silicon in aqueous alkaline solutions, e.g. NaOH or KOH solutions, is performed without electrical biasing. This is because the anodic biasing of silicon in alkaline solutions results in inactivation of the silicon, so that etching stops. Even in such aggressive media as KOH, silicon remains active only in a very narrow potential range more positive than corrosion potential Ecorr. Biasing of silicon in this range does not significantly affect the etching rate in comparison to silicon dissolution without biasing. Thus, except when etching in HF, anodic bias is not used for etching silicon.
Little data is available concerning the electrochemical behavior of semiconductors under cathodic bias. Gerisher and Mindt, Electrochim. Acta, 13, 1329, (1968) pointed out the feasibility of the cathodic decomposition of semiconductors. Additionally, they showed the reduction of such semiconductors as ZnO, CdO, CuS, etc. to their metal states.
It has also been shown by Glembocki et al., J. Electrochem. Soc., Vol. 132, 145-151 (1985), and by Seidel, in xe2x80x9cIntegrated Micro-Motion Systemsxe2x80x94Micromachining, Control and Applications,xe2x80x9d F. Harashima, Ed.; Elsevier Science Publishers, B.V., 1990 pp.51-68, that applying cathodic bias to silicon causes a decrease in the dissolution rate of n-type silicon and has almost no affect on the dissolution of p-type specimens. These experiments were carried out in KOH solutions in the potential range from xe2x88x926 to +0.5 V relative to the open circuit potential (OCP).
Accordingly, there is a need for an improved method for micromachining three-dimensional structures that is electrochemically controlled, more versatile, and safer to the environment.
The present invention seeks to provide an improved method for etching three-dimensional structures in semiconductor materials, which method enables greater control of the rate of the etching process, is applicable to a wider variety of semiconductor materials, and is environmentally safer than etching methods currently known and in use in the art.
There is thus provided, in accordance with a preferred embodiment of the invention, a process for etching a semiconductor material, comprising the steps of
(a) providing an electrochemical cell containing an etching electrolyte, the etching electrolyte being selected from the group of acidic electrolyte solutions, alkaline solutions, neutral solutions, and molten electrolytes;
(b) immersing the semiconductor material in the etching electrolyte, whereby at least one surface of the semiconductor material contacts the etching electrolyte;
(c) after step (b), negatively biasing the semiconductor material; and
(d) while negatively biasing the semiconductor material, illuminating at least part of the at least one surface of the semiconductor material which contacts the etching electrolyte with light selected from the group of ultraviolet, visible, and infrared light.
In one preferred embodiment of the invention, the etching electrolyte is selected from among acidic aqueous solutions, alkaline aqueous solutions, and neutral aqueous solutions. In another preferred embodiment of the invention, the etching electrolyte is a molten salt.
In another preferred embodiment of the invention, the semiconductor material is masked and patterned prior to immersion. In accordance with this preferred embodiment of the invention, the process of the invention comprises the steps of:
(i) providing an electrochemical cell containing an etching electrolyte, the etching electrolyte being selected from the group of acidic electrolyte solutions, alkaline solutions, neutral solutions, and molten electrolytes;
(ii) masking and patterning the semiconductor material with a masking material which is electrically insulating and less susceptible to the etching electrolyte than is the semiconductor material, whereby to provide a masked semiconductor material having at least one masked and at least one exposed surface;
(iii) immersing the masked semiconductor material in the etching electrolyte, whereby at least one exposed surface of the masked semiconductor material contacts the etching electrolyte;
(iv) after step (iii), negatively biasing the masked semiconductor material until the potential reaches a negative voltage value; and
(v) while negatively biasing the masked semiconductor material, illuminating at least part of the at least one exposed surface of the masked semiconductor material which contacts the etching electrolyte with light selected from the group of ultraviolet, visible, and infrared light.
In one preferred embodiment of the invention, step (ii) of the process comprises masking and patterning the semiconductor material with a masking material which is inert to the etching electrolyte.
In another preferred embodiment of the invention, step (b), or correspondingly, step (iii) of the process comprises immersing the semiconductor material in the etching electrolyte, until the open circuit potential of the semiconductor material reaches a steady state value. This preferred embodiment is especially preferred when the etching electrolyte is a strong alkaline solution, such as KOH. In yet another preferred embodiment of the invention, the semiconductor material is immersed in the etching electrolyte until the open circuit potential of the semiconductor material reaches a value of minus 1.1 V or a value more negative than minus 1.1 V, preferably from about minus 1.1 V to about minus 1.5 V, with respect to a Standard Calomel Electrode (SCE). This preferred embodiment is also especially preferred when the etching electrolyte is a strong alkaline solution, such as KOH.
In another preferred embodiment of the invention, step (c), or correspondingly, step (iv) of the process comprises negatively biasing the semiconductor material until the potential reaches a value of about minus 5 volts (SCE) or a value more negative than minus 5 volts (SCE).
In still another preferred embodiment of the invention, step (d), or correspondingly, step (v) of the process comprises illuminating the semiconductor material with light selected from the group of ultraviolet, visible, and infrared fight while negatively biasing said semiconductor material to a potential of about minus 5 volts (SCE) or a value more negative than minus 5 volts (SCE).
Thus, in an especially preferred embodiment of the invention, the process comprises:
(a) providing an electrochemical cell containing an etching electrolyte, the etching electrolyte being selected from the group of acidic electrolyte solutions, alkaline solutions, neutral solutions, and molten electrolytes;
(b) immersing the semiconductor material in the etching solution, whereby at least one surface of the semiconductor material contacts the etching electrolyte, until the open circuit potential of the semiconductor material reaches a steady state value;
(c) after step (b), negatively biasing the semiconductor material until the potential reaches minus 5 volts or more negative than minus 5 volts (SCE); and
(d) while negatively biasing the semiconductor material to a potential of minus 5 volts or more negative than minus 5 volts (SCE), illuminating at least part of the at least one surface of the semiconductor material which contacts the etching electrolyte with light selected from the group of ultraviolet, visible, and infrared light.
In another especially preferred embodiment of the invention, the process comprises:
(i) providing an electrochemical cell containing an etching electrolyte, the etching electrolyte being selected from the group of acidic electrolyte solutions, alkaline solutions, neutral solutions, and molten electrolytes;
(ii) masking and patterning the semiconductor material with a material which is inert to the etching electrolyte, whereby to provide a masked semiconductor material having at least one masked surface and at least one exposed surface;
(iii) after step (ii), immersing the masked semiconductor material in the etching electrolyte, whereby at least one of the at least one exposed surfaces of the masked semiconductor material contacts the etching electrolyte, until the open circuit potential of the masked semiconductor material reaches a steady state value;
(iv) after step (iii), negatively biasing the masked semiconductor material until the potential reaches minus 5 volts or more negative than minus 5 volts (SCE); and
(v) while negatively biasing the semiconductor material to a potential of minus 5 volts or more negative than minus 5 volts (SCE), illuminating at least part of the at least one exposed surface of the semiconductor material which contacts the etching electrolyte with light selected from the group of ultraviolet, visible, and infrared light.
In yet another especially preferred embodiment of the invention, the process comprises:
(a) providing an electrochemical cell containing an etching electrolyte, the etching electrolyte being selected from the group of acidic electrolyte solutions, alkaline solutions, neutral solutions, and molten electrolytes;
(b) immersing the semiconductor material in the etching electrolyte, whereby at least one surface of the semiconductor material contacts the etching electrolyte, until the open circuit potential of the semiconductor material reaches a value of about minus 1.1 V or a value more negative than minus 1.1 volts;
(c) after step (d), negatively biasing the semiconductor material until the potential reaches a value of minus 5 volts or a value more negative than xe2x88x925 volts (SCE); and
(d) while negatively biasing the semiconductor material to a potential of value minus 5 volts or a value more negative than minus 5 volts (SCE), illuminating at least part of the at least one surface of the semiconductor material which contacts the etching electrolyte with light selected from the group of ultraviolet, visible, and infrared light.
In still another especially preferred embodiment of the invention, the process comprises:
(i) providing an electrochemical cell containing an etching electrolyte, the etching electrolyte being selected from the group of acidic electrolyte solutions, alkaline solutions, neutral solutions, and molten electrolytes;
(ii) masking and patterning the semiconductor material with a material which is inert to the etching electrolyte, whereby to provide a masked semiconductor material having at least one masked surface and at least one exposed surface;
(iii) after step (ii), immersing the masked semiconductor material in the etching electrolyte, whereby at least one of the at least one exposed surfaces of the masked semiconductor material contacts the etching electrolyte, until the open circuit potential of the masked semiconductor material reaches a value of about minus 1.1 V or a value more negative than minus 1.1 volts;
(iv) after step (iii), negatively biasing the masked semiconductor material until the potential reaches minus 5 volts or more negative than xe2x88x925 volts (SCE); and
(v) while negatively biasing the masked semiconductor material to a potential of minus 5 volts or more negative than minus 5 volts (SCE), illuminating at least part of the at least one exposed surface of the semiconductor material which contacts the etching electrolyte with light selected from the group of ultraviolet, visible, and infrared light.
In a preferred embodiment of the invention, the illumination is supplied in such a manner that the intensity of the illumination, when measured in air at a point the same distance from the illumination source as the semiconductor material when the semiconductor material is immersed in the electrolytic cell, is at least 0.01 Watt/cm2.
In a preferred embodiment of the invention, the semiconductor material is selected from the group consisting of silicon, germanium, and semiconductors in the III-V and II-VI groups of semiconductors. By xe2x80x9cIII-V group of semiconductorsxe2x80x9d is meant semiconductors consisting substantially of (i) one or more elements from column IIIA of the periodic table and (ii) one or more elements from column VA of the periodic table, for example gallium arsenide. By xe2x80x9cII-VI group of semiconductorsxe2x80x9d is meant semiconductors consisting substantially of one or more elements from column IIA of the periodic table and one or more elements from column VIA of the periodic table, for example cadmium telluride.
In another preferred embodiment of the invention, the etching electrolyte is a solution containing a solute selected from the group consisting of alkali hydroxides, alkali halogenides and hydrogen halogenide acids, more preferably KOH, NaOH NaCl, and HF. In a more preferred embodiment, the solution is an aqueous solution. In an especially preferred embodiment, the aqueous solution is a neutral solution which is not harmful to laboratory or industrial equipment, and is not harmful to the touch.
In a preferred embodiment of the invention, the masked semiconductor material is illuminated with light with a wavelength between about 250 and about 1500 nm. In another preferred embodiment of the invention, the light is of intensity of at least 0.01 W/cm2.
In another preferred embodiment of the invention, the temperature of the solution is between about 25 and 90xc2x0 C., more preferably between about 50xc2x0 C. and about 90xc2x0 C.
In accordance with another preferred embodiment of the invention, there is provided an apparatus for electrochemically etching semiconductor materials, comprising an electrochemical cell, a holder for holding the semiconductor material and providing an electrical contact to the semiconductor material in the electrochemical cell, a counter electrode, a variable power source for variably biasing the semiconductor material when held in the electrochemical cell by the holder, and a light source, the electrochemical cell and the light source being mutually configured so as to enable direction of the light from the light source onto the semiconductor material held by the holder in the electrochemical cell.